While not limited thereto, the present invention is particularly adapted for high-temperature pressurized gasifiers for coal or other carbonaceous material. Such gasifiers usually comprise a cylindrical pressure vessel in which coal fines and gasifying agents (e.g., oxygen and steam) are reacted by partial oxidation into mainly 2 or 3 atom gases such as CO, H.sub.2, CO.sub.2 and H.sub.2 O. One such gasifier of this type is the slag bath generator in which the feed materials are injected through a number of nozzles onto a slag bath which covers the bottom of the pressure vessel and consists of the melted mineral constituents of the fuel supplied. Gasification proceeds at very high temperatures of from 1500.degree. C. to 2000.degree. C. and at pressures around 25 bar.
The pressurized metal casing, which is usually cylindrical in configuration, is protected from the high temperature of the combustion chamber by groups of tubes in the form of rafts of tubes, for instance, through which a coolant flows, the tubes providing vapor cooling or hydraulic cooling with condensation of steam. Groups of cooling tubes of this type experience severe mechanical stressing, even in normal operation, due to the high heat flow densities involved. Furthermore, faults in the tubing may lead to local overheating of the tube group and, hence, to its destruction. For instance, the nozzle flames within the combustion chamber may accidentally be deflected directly onto the tube group, resulting in local overheating and impairment of the strength of the material from which the tubes are formed, in which event the entire tube group or one of its tubes may rupture.
A layer of ceramic heat insulating material a few centimeters thick is normally provided between the cooling tubes and the vessel wall to protect the same against direct heat radiation. If the cooling system develops a fault, such as a rupture in a tube, the ceramic layer will eventually melt and expose the outer cylindrical metal casing to heat from the gas or flame, thereby creating a dangerous condition. This sequence of events may occur unknown to anyone since the combustion chamber is not visually accessible, with the result that in the last resort the pressure vessel itself is placed at risk.